The correct answer is A. Cement.
Cement is a binding material that is used in mortar, concrete, and other construction materials. It is made by heating limestone and clay to a
high temperature, and then grinding the resulting clinker into a fine powder. When mixed with water, cement forms a paste that hardens over time.Sand is a granular material that is used in mortar to provide a rough surface for the cement to bond to. It also helps to fill in voids and gaps in the mortar, which makes it stronger.
Surkhi (brick-dust) is a fine
powder that is made by grinding bricks. It is used in mortar to improve its workability and to reduce shrinkage.Cinder is a byproduct of coal combustion. It is a lightweight, porous material that is used in mortar to reduce its weight and to improve its insulation properties.
In conclusion, the binding material in mortar is cement. Sand, surkhi, and cinder are all used in mortar, but they do not serve as the binding material.